Why don’t we see certain traits in nature? If evolution is so innovative, if it’s powerful
enough to create this and this and this and whatever this is, etc… then… Why can’t humans grow wings and take to
the sky? Or why don’t fish have propellers? Why are there no 5-legged cats or giraffe-sized
chickens? Why do no animals have wheels? And why haven’t zebras evolved machine guns
to fight off lions? Today, instead of talking about incredible,
mind-blowing traits that evolution has created, like we’ve done so many times before, we’re
going to talk about the limits of evolution’s creativity, and why certain traits are impossible
to evolve. There is a myth in evolution that nature is
infinitely creative. It isn’t. Probably. By considering the reasons why certain things
can’t evolve we can learn a lot about how evolution actually does work. So yes, while these would all be awesome… I don’t make the rules, evolution does. … stupid evolution. I want a tank zebra. We begin in my childhood nightmares. The 1985 film Return To Oz was one of the
most terrifying films I ever accidentally flipped the channel to as a young nerd. This unofficial sequel to the 1939 film The
Wizard of Oz provided the fodder for more than one nightmare thanks in large part to
these: The Wheelers. There is something deeply unnatural about
a creature with wheels instead of hands or feet. Both because it’s extremely creepy, and
because it’s biologically impossible. The wheeled animal question is a canonical
case study when it comes to impossible things in evolution, or “forbidden phenotypes”. And remember, a phenotype is the observable
physical properties of an organism. There are organisms that roll up their entire
bodies to enable wheel-like rolling movement, like pangolins, spiders, tumbleweeds, and
roly-polies… or whatever you call these things where you’re from. Or maybe we could envision an animal rolling
around like the mulefa, a fictional species in the fantasy series His Dark Materials,
which hooks its feet into round seed pods and uses them as wheels. But acting like a wheel or using a found object
as a wheel is not the same as having wheels as body parts. No animal uses a rolling wheel body part to
move its body. And some of the reasons why are pretty obvious:
A structure capable of rolling continuously around an axle would be physically separated
from the body and therefore impossible to build in a developing animal. This would also make it impossible to deliver
nutrients and blood or nerve impulses to the wheeled appendage as well. But there are other less-obvious reasons animals
don’t have wheels. Which brings us here. To a Bronze Age foot soldier, the most terrifying
sight imaginable would’ve been enemy chariots rolling onto the battlefield. Manned by a driver and either an archer or
javelin thrower, the ability of these horse-drawn wheeled vehicles to move quickly across the
field of battle made the chariot the dominant shock and awe weapon of its time from Mesopotamia
to the Mediterranean. But by the 6th century AD, the chariot, along
with almost every other wheeled form of transportation, had basically disappeared between North Africa
to Central Asia. How could such a seemingly dominant technology
vanish? Because wheels had been replaced by camels. This happened for several reasons: The roads
originally laid down across the Roman empire had deteriorated. The skill and craftsmanship required to make
efficient wagons and carts had been slowly forgotten. But most simply, in this particular region
camels were just better and more efficient than wheels when it came to carrying stuff. The camel can travel farther, with less food
and water than a horse or ox. They can cross rivers and rough terrain easier
than a wheeled cart, and where a wagon requires a person to tend every two animals or so,
half a dozen fully loaded camels could be managed by one person. Likewise, Europeans were stunned to find no
wheeled vehicles used by the Aztecs, Incas, and other American indigenous cultures, even
in places where llamas were used as pack animals. Archaeological discoveries show us early American
cultures definitely invented wheels of their own, but beyond water wheels for
milling, or toys, they didn’t find wheels all that useful or necessary for their particular
terrain and environment. And this is how chariots and camels and creepy
Wizard of Oz sequels relate to evolution: Biological or cultural adaptations depend
on the environment in which they arise. The best solution to a problem depends on
the problem. While propellers and spinning blades are an
optimal way to move human-made craft through the water, the fins of fish are actually more
efficient at providing propulsion in most cases. So, there’s no fish with propellers, and
because we haven’t been able to match evolution’s aquatic creativity, we don’t have boats
powered by big tail fins. Likewise, the wheel, as a technology, isn’t
intrinsically better or more advanced than other ways of moving. The wheel only dominated in certain environments,
under certain conditions. And even if animals were capable of growing
wheels as body parts, in most environments and terrains they would probably work worse
than feet or hooves. Which brings us to a different kind of terrain
altogether… the fitness landscape. Here I don’t mean fitness like your ability
to run a mile or do pushups. In biology, fitness is essentially a score
that represents a trait’s ability to survive and reproduce. The wings of birds, bats and even pterosaurs,
are all modified structures of the arm, hand, and fingers. They are an example of convergent evolution,
where organisms that aren’t that closely related evolve similar features or phenotypes. You have to go pretty far back to find the
common ancestor between these winged creatures, but the wings of birds, bats, and pterosaurs
are all descended from an arm that is built essentially the same as yours or mine. One bone up here, two bones here, lots of
little bones here and long bones here. So theoretically, humans could evolve wings,
right? Actually, no. When a trait evolves, every stage of its evolution
pretty much has to provide an advantage, or at least not be harmful. Even if the final trait, like humans flying
with wings, would be super cool and give us lots of new advantages, we would have to be
able to get from this to wings in a way that every step is beneficial. You can’t evolve anything that reduces your
fitness. A fitness landscape is a way to look at a
lot of different variations and how they score versus one another. Each square represents a variation or genetic
possibility. The closer the squares, the more similar two
variations are, and the further the squares, the more different they are. The fitness of each genetic possibility is
represented by its height on the landscape. Here’s the problem. You can only ever move uphill, toward higher
fitness. There may be a highest peak, with the best
trait, on your landscape but you can’t get there because you’d have to travel down
into a valley first. Evolution is walking around this landscape
blind, it doesn’t plan or have foresight, so organisms often get stuck on top of these
little hills, called local optima. They’re as good as they can be without getting
worse. The human eye is a perfect example of a local
optimum. If the optic nerve and the eye’s blood vessels
ran behind the eye instead of through it, we wouldn’t have a blind spot in our vision. This is how octopus eyes are built and it’s
a much better way to make an eye. But we can’t jump all the way to that higher
fitness peak, or travel through a valley where we’d make our eyes worse in the meantime. Likewise, if we wanted to have wings like
birds, each stage of our evolution from arms to wings would have to provide a benefit. When the first birds were evolving from raptor-like
dinosaurs, they already had feathers for attracting mates. So they could use them to glide like flying
squirrels—that’s a fitness advantage. And each tweak and variation would help them
glide farther—more fitness improvements—all the way to powered flight… at the top of
a fitness hill. For humans, variations in our hands or arms
that gave us a small amount of gliding ability wouldn’t really improve our fitness right
away. But having dumb wing hands would have a lot
of costs when it came to things like using tools. So even if being able to truly fly would be
a huge improvement overall, those intermediate steps can’t evolve if they mean moving down
the fitness hill. And this is why zebras don’t have laser
turrets. Zebras would clearly benefit from evolving
defensive laser weapons to keep lions away, but they can’t because the intermediate
steps of evolution have to be helpful or at least not harmful. A laser turret or machine gun or bazooka is
only useful when it is complete. The intermediate, functionless laser organ
would just be hogging vital nutrients, a fitness loss. And sure, projectile weapons have evolved
in other animals. Like archerfish that hunt by spitting water,
or antlions that use sand as a projectile weapon. But if you’re prey, running away might just
work well enough, so you’re stuck at a local optimum without laser turrets. Evolution works like trying to make improvements
to an engine while the car is in the middle of a race. You can’t break your engine in order to
make it work better. Of course, I’m not willing to give up my
dream of flying so easily. Maybe there’s another way humans could grow
wings. Why can’t we just grow a new set of limbs? You can imagine this strategy working for
some bugs like millipedes. Entire body segments could be duplicated thanks
to mutations in genes controlling how the body develops, and poof: you’ve got a new
pair of legs to add to your 48 other pairs. Mutations in a class of body-patterning genes
called Hox genes have been linked to misshapen feet, hands, skulls, and even extra fingers
or toes. In very rare cases humans are even born with
extra limbs, like this baby who was born with three arms. But almost always, these duplicated limbs
don’t function, because the new limb also needs bones, joints, its own muscles and nerves,
and each of those duplications would require countless other mutations in other genes. That’s beyond unlikely. Evolving wheels instead of feet, or finding
a fish with a propeller, or modifying our hands into wings like a bat, is difficult
enough to be impossible. But at the end of the day there also has to
be a need to evolve - you don’t just get something because it’s cool. This is perhaps the worst car ever made in
the history of driving. The Trabant had a 25 hp two-stroke engine
closer to what’s in a lawnmower than a car, a plastic body, no fuel door, no rear seatbelts,
not even a turn signal indicator. Although their design changed little from
the late 1950s through 1990, when production ceased, more than 3 million were sold. That’s because this was one of the only
automobiles available in Communist East Germany. Before the fall of the Berlin Wall in 1989,
East Germany was a closed economy, meaning there was no competition from other car manufacturers
and no pressure to improve this vehicle. For natural selection to happen an organism
has to run into some challenge that impacts its survival, what we call a selection pressure. The East German lawnmower sedan never had
selection pressure from other automobiles forcing it to improve, so it continued to
sell well despite being very bad. Sauschlecht! Until humans experience a selection pressure
for gliding, we won’t start to evolve adaptations that lead towards flight. And sometimes selection pressures can be so
sudden or different, that there is no adaptation for evolution to even act on. The dodo had no adaptations for defending
itself, because it never encountered humans or other predators, and well, we all know
how that ended. But beyond all these principles of evolutionary
biology, one of the biggest limitations we face is physics. PHYYYYYYYSICCCCSSSSSS!!! The reason we don’t see gigantic land animals
like Mr. Longneck here any more, is because of two pesky bits of physics. One, gravity. You’re probably familiar with that one. Personally it’s how I stay so down to Earth. Heh. Two, the square-cube law. As an organism gets bigger, its volume increases
much faster than its surface area. So as something grows and gets more and more
volume, any process that depends on the amount of surface that you have will become less
and less efficient unless you change your shape to make more surface. This is the reason big complex organisms like
us are multicellular instead of 6-foot-tall single-celled amoeba blobs. That wouldn’t provide enough surface area
to exchange nutrients and waste and make energy for all of our big blobby volume. So instead our bodies are made of 37 trillion
cells, give or take. It’s the only way to have enough surface
area for all… thissss. Gravity and the square-cube law combined is
why here on Earth it’s unlikely we’d ever get land animals weighing more than 100 tons. It’s also the reason if you’re ever given
the choice of fighting one horse-sized duck or 100 duck-sized horses, you should pick
the fight versus a horse-sized duck because duck legs would snap like toothpicks under
the weight. And the limitations of physics are not just
something animals have to worry about. Calculations of how gravity affects the flow
of water in trees estimate the maximum height a tree could ever reach on Earth is 130 meters. And sure enough, the tallest tree we know
of… is just under 116 meters tall. However, a different physical environment
comes with different physical restrictions. Blue whales, the largest animals to ever live
on Earth, weigh up to 173 tons because the buoyancy provided by their watery environment
counteracts the gravity. Therefore, on different planets with lower
gravity or different atmospheric composition, we might be able to see bigger animals. For instance, 300 million years ago the concentration
of oxygen in Earth’s atmosphere was much higher than it is today (reaching as high
as 30%). And insects which don’t have lungs and circulatory
systems like we do—they breathe by gases just diffusing into their bodies—were able
to grow much larger than insects today. Dragonflies the size of birds? Not thankyou sir. For all of nature’s creativity, a few empty
spots remain on the tree of life where branches seemingly can’t grow. What we call impossible phenotypes, that as
far as we know have never arisen in the history of Earth. Like freshwater coral reefs, or birds that
give birth to live young. Or plant-eating snakes. I mean, there are thousands of species of
snakes! You’d think one would take advantage of
the food source that all other snakes are ignoring and go vegetarian! Snakes’ closest relatives, lizards, many
of them eat plants. I mean, making the switch from a meat eating
to plant diet worked for pandas. Well, barely. But maybe there’s something about how a
snake is built that prevents it from getting enough energy from plants. Of course maybe we just don’t know. For a long time it was believed that all spiders
were carnivorous, until we found one that eats plants. It’s difficult to say things are totally
impossible. Given more time, evolution may well produce
some of the things we’ve talked about. Imagine if alien evolutionary biologists studied
life on Earth 450 million years ago. Flowering plants, flying insects, animals
able to walk and breathe on land… these may have seemed like impossible phenotypes
back then, but all of them happened. So while there are limits to what evolution
can do in single lineages, like humans growing wings or animals with wheels, we don’t really
know where the boundaries of evolution are as a whole. Like… what about animals with an odd number
of limbs? I don’t mean three limbs like Simone’s
super cute dog Scraps. A three legged dog is a four legged animal
that lost a limb due to an accident or medical issue. Or a 5-limbed animal like a starfish—radial
symmetry removes many of the limitations that would prevent odd numbers of limbs. I mean a three-limbed animal like a dolphin. Think about it. How many “legs” does a dolphin or whale
have? Their fins have two lobes, and their prehistoric
ancestors walked on all fours, but for all intents and purposes couldn’t we consider
a dolphin an animal that evolved to move with three limbs? Does that mean kangaroos and their powerful
tails, or monkeys with prehensile tails are, functionally speaking, 5-limbed animals? Are snakes one-limbed animals? In these cases, although they are rare, there
was an evolutionary path that allowed an odd number of limbs or limb-like appendages to
arise from even-numbered ancestors… that’s a pretty remarkable leap. For all the rules and impossibilities we’ve
talked about, the things evolution has been able to mold and create are pretty impressive. Instead of looking at what nature hasn’t
built, maybe we should marvel at all that has been. Stay curious. We’re still not gonna grow
wings though.
What happened here?
They didn't mention that the energy required to fire a laser is far more than a biological animal could realistically store in it's body.